176 



ZOOLOGY 



SECT. 



of which presents a broad, spiral flange. The young Elasmobranch 

 goes through its development enclosed in the shell, until it is fully 

 formed, when it escapes by rupturing the latter. In the viviparous 

 forms the ovum undergoes its development in the uterus, in which 

 in most cases it lies free except in some Mustelidse and 

 Carchariidae, in which there is a close connection between the yolk- 

 sac of the embryo and the wall of the uterus, folds of the former 

 interdigitating with folds of the latter, and nourishment being thus 

 conveyed from the vascular system of the mother to that of the 

 foetus by diffusion. In some of the viviparous forms a distinct 



though very delicate shell, some- 

 times having rudiments of the fila- 

 ments, is formed, and is thrown off 

 in the uterus. In the genera Rhino- 

 batus and TrygonorJiina, which are 

 both viviparous, each shell encloses 

 not one egg, but three or four. 

 Laemargus is said to differ from all 

 the rest of the Elasmobranchii in 

 having the ova fertilised after they 

 have been deposited, as well as in 

 the small size of the ova. 



Development. Segmentation is 

 meroblastic, 1 being confined to the 

 germinal disc, which, before divid- 

 ing, exhibits amoeboid movements. 

 While segmentation is going on in 

 the germinal disc there appear a 

 number of nuclei, the source of 

 which is not certain, in the substance 

 of the yolk. When segmentation is 

 complete, the blastoderm appears as 

 a lens-shaped disc, thicker at one 

 end the embryonic end. It is found 

 to consist of two layers of cells 

 an upper layer in a single^ stratum, and a lower layer several 

 cells deep. A segmentation-cavity appears early among the cells 

 of the lower layer ; the lower-layer cells afterwards disappear 

 from the floor of this, the cavity then coming to rest directly on the 

 yolk. 



An in-folding (Fig. 850) now begins at the thickened embryonic 

 edge of the blastoderm, which here becomes continuous with the 

 cells of the lower layer. The cavity (al), at first very small, 

 formed below this in-folding is the rudiment of the archenteron, 

 and the cells lining this cavity above, which form a definite layer, 

 partly derived from the in-folded ectoderm, partly from the cells 



1 Except in one species of Cestracion. 



Fia. 849. Egg-case of Cestracion 

 galeatus. (After Waite.) 



